I described what I was working on, what research I did on those industries and that I was looking to better understand the existing problems that might fit what I was interested in.
Some conversations did not lead anywhere, other times I got excellent, tactical, advice. That gave me more information and ideas, until I landed on my current project.
The reason this approach worked for me this time (as opposed to the other times I've tried this) is that I had a direction I was presenting and I did a bunch of research to get educated about some of these industries. I don't mean I studied it for some long time, but reading related subreddits, searching for conferences and looking up sponsor companies from that, learning some terms specific to those industries, etc. helped to have a better conversation.
While YMMV, getting warm intros to smart and successful people for a 15-20 minute phone conversation will expose you to what you don't know yet. This will result in you asking better questions, which can lead to more ideas.
Though I did start out with an idea that I flushed out by talking to people in the industry.
Would love to hear more about your experience, let me know if you have time to compare notes.
Let us have it! And please add your email in profile
Another strategy is to look for large problems in your industry that have been solved and then try to find analogous problems in other industries where the same or a similar solution could work.
For our startup, my co-founder and I went and talked to friends and contacts of mine in medicine and generated a list of 15 or so things we thought were everyday pain points for many individuals. We then shelled out some money and went to an industry conference to field test them. It turns out that all of them were being actively worked on, but while we were exploring them we stumbled upon another idea which is now the basis for our startup.